Introduction / Context:
Interfacing different logic families requires checking both logic-level compatibility (VIH/VIL versus VOH/VOL) and drive capabilities (fanout, input currents). Blanket statements about “no danger” are risky. This question probes recognition of level compatibility nuances among 74HC, 74HCT, TTL, ALS, and LSTTL families.
Given Data / Assumptions:
- 74HC CMOS has CMOS-level VIH requirements (~0.7*VCC), not TTL-compatible.
- 74HCT is TTL-compatible (HCT “T” = TTL input thresholds).
- 74ALS/74LSTTL are TTL families with defined VOH/VOL; VOH(min) may be ~2.4 V.
- Device currents and fanout must be respected; some TTL outputs source weak HIGH current.
Concept / Approach:
Safe interfacing demands that VOH/VOL of the driving family meet VIH/VIL of the receiving family with comfortable noise margins. 74HC inputs often need VIH near 3.5 V at VCC=5 V; a TTL output VOH(min) at ~2.4 V will not reliably satisfy that. 74HCT fixes this by using TTL thresholds. Therefore, the absolute claim of “no danger” is wrong.
Step-by-Step Solution:
Compare TTL VOH(min) (~2.4 V) with 74HC VIH(min) (~0.7*5 V = 3.5 V) → incompatibility risk.Recognize 74HCT inputs are TTL-compatible → OK with TTL/ALS/LSTTL drivers.Infer: Interfacing is conditionally safe, not “always with no danger.”
Verification / Alternative check:
Check datasheets for VIH/VIL/VOH/VOL and ensure positive noise margins in both directions.
Why Other Options Are Wrong:
Correct: Would ignore documented threshold mismatches.Only correct if using 74HCT instead of 74HC: This is a helpful guideline but not universally sufficient; drive currents and fanout still matter.Only correct at very low frequencies: Frequency does not fix DC level mismatches.
Common Pitfalls:
Assuming “HC = HCT” for thresholds.Ignoring fanout and input leakage/drive limits.
Final Answer:
Incorrect
Discussion & Comments